(1) Field of the Invention
The present invention relates to aluminum borate whiskers and a process for the preparation thereof.
The whisker of the present invention is excellent in chemical resistance, heat resistance, neutron-absorbing capacity and electrical insulating property and is valuable as a heat-insulating material, a heat-resistant material and a corrosion-resistant material and also as a reinforcer for thermoplastic resins, thermosetting resins, cements, ceramics and metals.
(2) Description of the Prior Art
Recently, ceramic whiskers having excellent thermal and mechanical properties have been manufactured on an industrial scale and investigations have been made on these ceramic whiskers. The ceramic whiskers are roughly divided into non-oxide and oxide types. SiC, Si.sub.3 N.sub.4 and the like belong to the former type, and CaSO.sub.4, K.sub.2 O.6TiO.sub.2 (Japanese Patent Publication No. 25157/80) and 2MgO.B.sub.2 O.sub.3 (Japanese Patent Application Laid-Open Specification No. 204697/85) belong to the latter type.
Non-oxide type whiskers generally have a high melting point (higher than 1900.degree. C.) and have an excellent heat resistance, but since the atmosphere should be adjusted at the production step, the equipment and material costs are increased and uses of obtained whiskers are limited.
In contrast, the melting points of oxide type whiskers are not so high (1000.degree. to 1600.degree. C.) as those of non-oxide type whiskers and the heat resistance is inferior, but since oxide type whiskers can be prepared in the open air by the liquid phase method using a flux, the equipment and material costs are low. Accordingly, oxide type whiskers can be used in the general-purpose fields where a high heat resistance is not required, for example, as a reinforcer for plastics and as a reinforcer for aluminum.
The phase diagram of a boric acid salt of aluminum generally called aluminum borate has already been completed [P. J. M. Gielisse, Nature, 4836, 69-70 (1962)], and the presence of 9Al.sub.2 O.sub.3.2B.sub.2 O.sub.3 and 2Al.sub.2 O.sub.3.B.sub.2 O.sub.3 as compounds have been clarified. However, whiskers of these compounds have not been provided. Although synthesis of whiskers of these compounds was tried several times in the past, all of the obtained whiskers had a composition of aluminum and boron not shown in the phase diagram.
For example, from the specification of U.S. Pat. No. 3,080,242, it is known that whiskers having compositions of 3Al.sub.2 O.sub.3.B.sub.2 O.sub.3 and 9Al.sub.2 O.sub.3.B.sub.2 O.sub.3 are obtained by heating aluminum hydroxide and boron oxide and gradually cooling the heated mixture. From the report published by Bureau of Mines, the Department of the Interior, U.S.A., it is known that whiskers having a composition of 2.5Al.sub.2 O.sub.3.B.sub.2 O.sub.3 are obtained by heating aluminum oxide, sodium tetraborate and lithium chloride at 1200.degree. C. and whiskers having a composition of 4.8Al.sub.2 O.sub.3.B.sub.2 O.sub.3 are obtained by heating aluminum oxide, sodium tetraborate and boron oxide [R. C. Johnson, "Bureau of Mines Report of Investigations, 6675" (1965)].
Furthermore, with respect to whiskers of the latter type, the specification of U.S. Pat. No. 3,350,166 teaches that whiskers having a composition of 4Al.sub.2 O.sub.3.B.sub.2 O.sub.3 are obtained by the gas phase reaction process comprising passing steam through aluminum fluoride and boron oxide in the gaseous state at 1000.degree. to 1400.degree. C.
Oxide type whiskers such as CaSO.sub.4, K.sub.2 O.6TiO.sub.2 and 2MgO.B.sub.2 O.sub.3 can be prepared at low costs by the liquid phase process and expansion of uses is expected, but they are defective in that the chemical resistance, especially the acid resistance, is inferior and when they are used as reinforcers, it is apprehended that a problem will arise in connection with the long-period reliability.
Aluminum borate whiskers have already been synthesized, as pointed out hereinbefore, and molecular formulae of these whiskers are 3Al.sub.2 O.sub.3.B.sub.2 O.sub.3, 2.5Al.sub.2 O.sub.3.B.sub.2 O.sub.3, 4Al.sub.2 O.sub.3.B.sub.2 O.sub.3, 4.8Al.sub.2 O.sub.3.B.sub.2 O.sub.3 and 9Al.sub.2 O.sub.3.B.sub.2 O.sub.3, but whiskers of aluminum borate having a composition of 9Al.sub.2 O.sub.3.2B.sub.2 O.sub.3, or 2Al.sub.2 O.sub.3.B.sub.2 O.sub.3 have not been synthesized.
The whisker is a needle-shaped single crystal having a high aspect ratio, which has no grain boundary and a smaller number of defects than a polycrystal, and therefore, the whisker is characterized in that the mechanical strength is very high. However, if the size is increased, the number of defects is increased and in some cases, a polycrystal comprising single crystals aggregating along the fiber axis is formed and the strength is drastically reduced, with the result that the significance of use of the whisker is lost. More specifically, it was reported that a whisker having a fiber diameter of about 1 .mu.m has a tensile strength of 5000 MPa but if the fiber diameter is increased to 5 .mu.m, the strength is reduced to 700 MPa. The above-mentioned aluminum borate whiskers synthesized in the past have a fiber diameter of about 5 to about 100 .mu.m, which is included in the region where the mechanical strength is drastically reduced, and when the whisker is used as a reinforcer, a whisker having a fiber diameter of about 1 .mu.m is desirable.